django saving modelform with MultipleChoiceField

Question

I'm using MySQL

my table model :

class Offer(models.Model):
...
    additional=models.CharField('Dodatkowe informacje',max_length="100",choices=ADDITIONAL_CHOICES.items(),blank=True)
...

my form: class OfferForm(forms.ModelForm): additional=forms.MultipleChoiceField(label='dodatkowe informacje',choices=ADDITIONAL_CHOICES.items(),required=False) class Meta: model=Offer

while saving form i receive this error : (1064, "You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near '), description = 'sagas', local_h = '12.3450000000', local_w = '45.0940000' at line 1")

sql looks like this: 'UPDATE offers_offer SET cat_id = 2, type = 2, estate_type = 3, created = \'2009-03-17 01:58:31\', area = \'3245.000\', city = \'sdfghksdj\', district = \'gksdgj\', price = \'235252.000\', additional = ("\'1\'",), description = \'sagas\', local_h = \'12.3450000000\', local_w = \'45.0940000000\', gmapimg = \'\', title = \'dgsg\', meta = \'sdgs\', publish = 1 WHERE offers_offer.id = 3 '

when I used postgre I didn't this error :/ now I work with MySQL and have no ideas maybe i should use other fild type than vchar ?